The present invention relates to a method for making polycarbonates by the melt condensation of a diarylcarbonate and an aromatic bis hydroxy compound. More particularly, the present invention relates to the use of a tetraorganophosphonium carboxylate salt, such as tetrabutylphosphonium acetate, as a catalyst, to initiate rapid carbonate transesterification.
Methods for making polycarbonate by a solventless melt condensation reaction between diphenyl carbonate and bisphenol A are shown by Sakashita et al, U.S. Pat. No. 5,026,817. In one procedure, Sakashita et al use reactants having a particular range of hydrolyzable chlorine. In another method, a mixed catalyst is used, such as a nitrogen containing basic compound and an aromatic hydroxy salt of an alkali metal or alkaline earth metal. It has been found, however, that during such melt polymerization reactions, nitrogen containing basic compounds, such as tetramethylammonium hydroxide, can decompose to produce a variety of products including amines which can contaminate the final polymer. As a result of said decomposition, a second catalyst, such as sodium hydroxide is often employed as a finishing catalyst.
A further method for making polycarbonates by melt transesterification of diphenyl carbonate with 2,2-bis(4-hydroxyphenyl)propane is shown by Yangsoo Kim et al, Industrial Engineering Chemistry Research, 1992, 31, 2118-2127. Yangsoo Kim et al employ a lithium hydroxide catalyst.
Although alkali metal hydroxides have been found effective either as a finishing catalyst or as a condensation catalyst, these materials are carried into the final resin which can be adversely affected.
Volatile basic catalysts, such as organic nitrogen compounds, for example organic amines, also have been evaluated as condensation catalysts. Although such organic compounds can be readily removed from the polycarbonate resin at the termination of the condensation reaction, color bodies are often generated. In addition, even trace amounts of amines have been found to have a deleterious affect on the long term stability of the polymer. It would be desirable therefore, to provide organic compounds useful as catalysts for effecting the condensation between a diaryl carbonate and an aromatic bis hydroxy compound under melt conditions which are readily removable or which do not adversely affect the resulting polycarbonate resin.